The use of complementary and alternative medical (CAM) therapies, including botanical medicines, is increasing continuously. Botanical medicines are of particular importance, since these natural preparations may cause clinically important drug interactions with prescription drugs, in particular the antiretroviral drugs that have become the principal medical treatment for individuals with HIV infection and AIDS. This population commonly uses additional CAM treatments, including botanical medicines. Garlic is commonly taken by patients with HIV, and garlic has been shown to interact with saquinavir. The broad objective of this project is to develop and test research models to enhance mechanistic understanding of interactions between botanicals and antiretrovirals, such that clinically important interactions can be anticipated and prevented. We propose a combined experimental and clinical approach to evaluating the influence of garlic, and of selected chemical components of garlic, on the processes regulating disposition of the majority of antiretroviral agents (metabolism by CYP3A and transport by P-glycoprotein), and on the pharmacokinetics of two representative antiretrovirals, ritonavir and saquinavir. There are two specific aims. The experimental component will assess the effect of two garlic preparations (standardized garlic and aged garlic extract), and several individual chemical components, in the following systems: inhibitory actions on human CYP3A (and other CYPs) in human liver microsomes; induction of CYP3A in primary human hepatocyte culture; inhibition of P-gp transport in Caco-2 transwell cultures; and induction of P-gp expression in LS180V transwell cell cultures. In clinical studies of healthy human volunteers, we will study the effect of 14 days of exposure to standardized garlic, aged garlic extract, and placebo on: the clearance of oral midazolam, a CYP3A probe substrate; the clearance of oral fexofenadine, a probe P-gp substrate; and the pharmacokinetics of ritonavir and saquinavir, representative antiretrovirals dependent on CYP3A and P-gp for disposition and clearance. Kinetics of these compounds are determined prior to, during, and after garlic exposure. The work will provide immediately applicable clinical data on garlic interactions with antiretrovirals, as well as mechanistic data identifying the interaction process and its predictability from in vitro models.